Random Access Procedure
A random access (RA) procedure is a key function in a cellular system. In LTE, a wireless device, e.g., a user equipment (UE), that would like to access the network initiates the random access procedure by transmitting a preamble (e.g., Msg1) in the uplink on the Physical Random Access Channel (PRACH). A Next Generation NodeB (gNB) or TRP (Transmission and Reception Point i.e. a base station, access node) receiving the preamble and detecting the random-access attempt will respond in the downlink by transmitting a random access response (RAR, e.g., Msg2). The RAR carries an uplink scheduling grant for the UE to continue the procedure by transmitting a following subsequent message in the uplink (e.g., Msg3) for terminal identification. A similar procedure is envisioned for new radio (NR). For example, FIG. 1 illustrates an example of an initial access procedure considered for NR.
Before transmission of the PRACH preamble, the UE receives both a set of synchronization signals and configuration parameters on a broadcast channel in an SS-block (e.g., NR-PSS, NR-SSS, NR-PBCH), possibly complemented with configuration parameters received on yet another channel.
A possible PRACH preamble design for NR is described in R1-1609671, “NR PRACH preamble design”, 3GPP TSG-RAN WG1 #86bis, Lisbon, Portugal, Sep. 10-14, 2016, as also illustrated by formats 2 to 5 in FIG. 2 (PRACH preamble formats). This PRACH format is based on repeating the same PRACH sequence (or PRACH OFDM symbol) without a cyclic prefix (CP) between the repetitions, such that one PRACH OFDM symbol acts as a cyclic prefix for the next PRACH OFDM symbol.
FIG. 2 illustrates six formats with different lengths of the PRACH preamble such that they can be used for different coverage situations or for different receiver beamforming sweep. The length of each format may be changed depending of sub-carrier spacing. Here, a slot is used as time unit on the horizontal axis with 14 PUSCH OFDM symbols in each slot.
The mapping from one SS-block to a set of PRACH preambles depends on number of PRACH preambles associated with each SS-block. Here, the maximum number of SS-blocks, L, will depend on the carrier frequency according to agreements in 3GPP RAN1#88bis where the maximum number of SS-blocks is to be selected between 1 and 64:
Agreements:
The considered maximum number of SS-blocks, L, within SS burst set for different frequency ranges are
For frequency range up to 3 GHz, the maximum number of SS-blocks, L, within SS burst set is [1, 2, 4]
For frequency range from 3 GHz to 6 GHz, the maximum number of SS-blocks, L, within SS burst set is [4, 8]
For frequency range from 6 GHz to 52.6 GHz, the maximum number of SS-blocks, L, within SS burst set is [64]
The way the value of L is reflected in specification is FFS
Aforementioned values are to be used to facilitate the NR initial access design and evaluate the specification impact
Possibility of having unified frequency agnostic signaling design is not precluded
It is further proposed to be possible to indicate that not every SS block is actually transmitted:
Working Assumptions:
UE-specific RRC signaling with full bitmap can be used for indicating the actually transmitted SS blocks for both sub 6 GHz and over 6 GHz cases
The actually transmitted SS blocks is indicated in RMSI for both sub 6 GHz and over 6 GHz cases
Indication is in compressed form in above 6 GHz case, and an indication method is down-selected from following alternatives
Alt. 1: Group-Bitmap+Bitmap in Group
A Group is defined as consecutive SS/PBCH blocks
Bitmap in Group can indicate which SS/PBCH block is actually transmitted within a Group, each Group has the same pattern of SS/PBCH block transmission, and Group-Bitmap can indicate which Group is actually transmitted
E.g., [8]+[8] bits in case of 8 Groups and 8 SS/PBCH blocks per Group
Alt. 2: Group-Bitmap+The number of actually transmitted SS/PBCH block in Group (with fixed starting index of SS/PBCH block)
A Group is defined as consecutive SS/PBCH blocks
Group-Bitmap can indicate which Group is actually transmitted, SS/PBCH blocks within a Group are logically consecutive, the number of actually transmitted SS/PBCH block indicates how many logically consecutive SS/PBCH blocks are actually transmitted starting from the first index, and the number is commonly applied to all transmitted Groups
E.g., [8]+[3] bits in case of 8 Groups and 8 SS/PBCH blocks per Group
Alt. 3: Bitmap in Group+The number of actually transmitted Groups (with fixed starting index of Group)
A Group is defined as consecutive SS/PBCH blocks
Bitmap in Group can indicate which SS/PBCH block is actually transmitted within a Group, each Group has the same pattern of SS/PBCH block transmission, and the number of actually transmitted Groups indicates how many consecutive Groups are actually transmitted starting from the first Group
E.g., [8]+[3] bits in case of 8 Groups and 8 SS/PBCH blocks per Group
Alt. 4: Group-Bitmap+The number of actually transmitted SS/PBCH block in each Group
A Group is defined as consecutive SS/PBCH blocks
Group-Bitmap can indicate which Group is actually transmitted, SS/PBCH blocks within a Group are logically consecutive, and the number of actually transmitted SS/PBCH block for each Group indicates how many logically consecutive SS/PBCH blocks are actually transmitted starting from the first index
Minimum [8]+[3] bits, maximum [8]+[3]*[8] bits in case of 8 Groups and 8 SS/PBCH blocks per Group
Alt. 5: The number of actually transmitted SS/PBCH blocks+starting index+gap between two consecutive SS/PBCH blocks
[6]+[6]+[6] bits
Alt. 6: Group-Bitmap
A Group is defined as consecutive SS/PBCH blocks
Group-Bitmap can indicate which Group is actually transmitted, and all SS/PBCH blocks within a transmitted Group are actually transmitted
E.g., [8] bits in case of 8 Groups and 8 SS/PBCH blocks per Group
Other alternatives are not precluded
Indicated resources are reserved for actually transmitted SS blocks
Data channels are rate matched around actually transmitted SS blocks
Working Assumption:
For indication in RMSI:
Alt. 1: Group-Bitmap(8 bits)+Bitmap in Group (8 bits)
A Group is defined as consecutive SS/PBCH blocks
Bitmap in Group can indicate which SS/PBCH block is actually transmitted within a Group, where each Group has the same pattern of SS/PBCH block transmission, and Group-Bitmap can indicate which Group is actually transmitted
Agreements:
Confirm working assumption of:
UE-specific RRC signaling with full bitmap can be used for indicating the actually transmitted SS blocks for both sub 6 GHz and over 6 GHz cases
The actually transmitted SS blocks is indicated in RMSI for both sub 6 GHz and over 6 GHz cases
Indication is in compressed form in above 6 GHz case
Indicated resources are reserved for actually transmitted SS blocks
Data channels are rate matched around actually transmitted SS blocks
The number of configured PRACH preambles in each cell was defined to be 64 in LTE (e.g., 3GPP 36.211 section 5.7.2). These PRACH preambles are shared between contention based and non-contention based access.